Size-activity threshold of titanium dioxide-supported Cu cluster in CO oxidation

Development of non-noble metal cluster catalysts, aiming at concurrently high activity and stability, for emission control systems has been challenging because of sintering and overcoating of clusters on the support. In this work, we reported the role of well-dispersed copper nanoclusters supported on TiO2 in CO oxidation under industrially relevant operating conditions. The catalyst containing 0.15 wt% Cu on TiO2 (0.15 CT) exhibited a high dispersion (59.1%), a large specific surface area (381 m2/gCu), a small particle size (1.77 nm), and abundant active sites (75.8% Cu2O). The CO oxidation activity measured by the turnover frequency (TOF) was found to be enhanced from 0.60 × 10−3 to 3.22 × 10−3 molCO·molCu−1·s−1 as the copper loading decreased from 5 to 0.15 wt%. A CO conversion of approximately 60% was still observed in the supported cluster catalyst with a Cu loading of 5 wt% at 240 °C. No deactivation was observed for catalysts with low copper loading (0.15 and 0.30 CT) after 8 h of time-on-stream, which compares favorably with less stable Au cluster-based catalysts reported in the literature. In contrast, catalysts with high copper loading (0.75 and 5 CT) showed deactivation over time, which was ascribed to the increase in copper particle size due to metal cluster agglomeration. This study elucidated the size-activity threshold of TiO2-supported Cu cluster catalysts. It also demonstrated the potential of the supported Cu cluster catalyst at a typical temperature range of diesel engines at light-load. The supported Cu cluster catalyst could be a promising alternative to noble metal cluster catalysts for emission control systems. [Display omitted] •Cu6 nanoclusters supported on TiO2 were successfully synthesized.•0.15 wt% of Cu gave the highest TOF and metal dispersion.•0.30 wt% of Cu was found to be the size-activity threshold for CO oxidation.•The Cu loading above 0.30 wt% showed deactivation during the reaction.•CO oxidation over Cu6 nanoclusters supported on TiO2 is surface reaction-limited. Cu clusters show superior catalytic activity in CO oxidation than Cu nanoparticles, which is strongly dependent on the degree of dispersion on the support material.